Bovine Respiratory Disease Complex (BRDC) consists of multiple microbial pathogens and contributes to substantial economic loss to the cattle industry. Treatment costs including both preventative vaccination and medication following an outbreak are estimated to be near $4 billion per year (Griffin, 1997). Adding to the economic impact is the associated loss in performance observed in animals diagnosed with BRDC; with measurable losses to average daily gain, body weight at harvest, and beef quality grade (Babcock, 2010). Reports on specific monetary loss associated with decreased performance vary; likely due to varying case definitions of BRDC, but are estimated between $40 (Fulton, 2002) and almost $300 (Duff and Gaylean 2011) per animal. The performance loss has also been shown to significantly increase with the number of times an animal requires treatment for BRDC (Fulton, 2002).
Histophilus somni (formerly Haemophilus somnus) has been identified as a key contributor to BRDC (Duff and Gaylean 2011). This gram-negative pleomorphic coccobacillus belonging to the family Pasterellacea (Korczak, 2004) makes up part of the normal microbiota of the upper respiratory and urogenital tracts in cattle, sheep, and other ruminants (Ward, 2006). It is closely related to other bovine pathogens including Pasteurella multocida and Mannheimia haemolytica (both of which are also associated with BRDC) as well as the human pathogens Haemophilus ducreyi and Haemophilus influenzae (Challacombe, 2007).
Estimates place the isolation rate of H. somni from the upper respiratory tract of healthy calves as high as 50% with no clinical manifestations of disease; however animals diagnosed with BRDC have an even higher isolation rate for the bacteria (Griffin, 2010). Under stressful conditions or states of immunosuppression, H. somni may colonize the lower respiratory tract, endocardium, or central nervous system and has been identified as an etiological agent in diverse diseases such as pneumonia, endocarditis, arthritis, abortion, septicemia, and thromboembolic meningoencephalitis (TEME) (Ward, 2006).
At the time of slaughter, less than 15% of animals receiving proper treatment for BRDC (preventative vaccinations and appropriate antibiotics during an outbreak) show signs of lung lesions and these lesions involve less than 5% of the total lung (Griffin, 2010). Conversely, 50% of animals not receiving proper care display lung lesions at the time of slaughter, and these lesions may involve 15% or more of the total lung (Griffin, 2010). In one field study of over 10,000 animals, 459 calves (4.6%) died from disease of one form or another. Of the mortalities in the study, 279 (60.8%) were shown to be related to respiratory ailments, and of those with respiratory infections, 226 (81.0%) were associated with H. somni related pneumonia, pleurisy, or abscesses (Ribble, 1988). While antibiotic treatment may be successful in response to an H. somni infection, an increasing prevalence of antibiotic resistant field isolates is of concern (Duff and Gaylean, 2011). Preventative care by vaccination would be preferred as it is proactive rather than reactive and is much more cost effective.
Many H. somni vaccines are currently available from various animal health companies; however these vaccines are predominantly composed of killed bacterins and were licensed over thirty years ago with an aim in preventing TEME. The use of these bacterin vaccines has been effective against TEME, however has been shown to have neutral or even negative effects on respiratory disease in feedlot cattle. Negative side-effects include IgE induced anaphylactic shock and interactions when calves infected with Bovine Respiratory Syncytial Virus (BRSV) are vaccinated (Griffin, 2010). The decrease in prevalence of TEME and the emergence of H. somni related pneumonia in the US and myocarditis in Canada beginning in the late 1980's, have led to a need for further investigation of efficacious antigens for vaccine production (O'Toole, 2009).
H. somni related pneumonia is an economically significant condition for the beef and dairy industries. There is little evidence of field efficacy in currently available vaccines, so the need for research into next generation vaccine products is warranted.